1. Field of the Invention
The present invention relates to grating-tuned external cavity lasers and more particularly to a method and apparatus for generating a continuously-tunable, low-noise laser beam in a grating-tuned external cavity laser.
2. Description of Related Art
Grating-tuned external cavity lasers produce continuously-tunable laser beams consisting of light with high coherence and very narrow linewidth. To obtain high coherence and narrow linewidth, a grating is generally employed to disperse the emission from a light source or gain element, and feed it back to the gain medium at a wavelength selected by a tuning device. Tunable laser beams can be produced either by rotating a grating in a Littrow-type arrangement, or a reflector in a Littman-type configuration. Littman-type tunable laser systems are described in the publications, xe2x80x9cSpectrally Narrow Pulse Dye Laser Without Beam Expander,xe2x80x9d by Michael G. Littman and Harold J. Metcalf, Applied Optics, Vol. 17, No. 14, pages 2224-2227, Jul. 15, 1978, and xe2x80x9cNarrowband Operation Of A Pulsed Dye Laser Without Intracavity Beam Expansionxe2x80x9d by l. Shoshan, N. N. Dannon, and U. P. Oppenheim, Journal of Applied Physics, Vol. 48, pages 4495-4497, 1977. A single-longitudinal-mode (very narrow linewidth) frequency tunable pulsed dye laser was described in the publication, xe2x80x9cSingle-Mode Pulsed Tunable Dye Laser,xe2x80x9d by M. G. Littman, Optics Letters, Vol. 23, pages 138-140, 1978. This single-longitudinal mode laser provides a foundation for producing tunable narrow-bandwidth lasers.
FIG. 1 shows a prior art grating-tuned external cavity laser capable of producing a laser beam which is tunable over a broad range of wavelengths by rotation of a tuning reflector. Laser system 100 comprises pivot 102, base 104, plane reflector 106, gain medium 108, diffraction grating 110, tuning reflector 112, rotatable unit 114, output laser beam 116 and first-order diffracted radiation 118.
In the prior art system of FIG. 1, a proximal end of rotatable unit 114 is pivotably connected to base 104 by pivot 102. Tuning reflector 112 is mounted on rotatable unit 114 forming an acute angle with respect to diffraction grating 110, which is mounted on an upper surface of base 104. Plane reflector 106 and gain medium 108 are mounted on base 104 and are disposed to produce a laser beam which is incident on diffraction grating 110 at a grazing angle, thereby generating output laser beam 116 and first-order diffracted radiation 118.
In operation, rotating arm 114 pivots around pivot 102 such that tuning reflector 112 moves relative to diffraction grating 110. Plane reflector 106 and gain element 108 generate a laser beam which is incident on diffraction grating 110 at a grazing angle. Part of this laser beam is reflected as output laser beam 116 and exits laser system 100. The rest of the laser beam incident on diffraction grating 110 is diffracted and reflected to generate a light radiation pattern which includes first-order diffracted radiation 118. First-order diffracted radiation 118 retro-reflects off tuning reflector 112 and is again incident on diffraction grating 110.
Upon further diffraction and reflection by diffraction grating 110, a portion of first-order diffracted radiation 118 enters gain element 108 and plane reflector 106, thereby forming an external feedback laser cavity for laser system 100. The wavelength of output laser beam 116 depends on the angle formed by grating surface 110 and the reflective surface of tuning reflector 112, which may be adjusted by pivoting rotatable unit 114 around pivot 102. Consequently, the wavelength of output laser beam 116 may be tuned by pivoting rotatable unit 114 around pivot 102. Accurate positioning of pivot 102 enables mode-hop-free, continuous tuning of output laser beam 116 over the entire emission band of gain element 108.
A laser system similar to the prior art system shown in FIG. 1 is described in the publication, xe2x80x9cNovel Geometry for Single-Mode Scanning of Tunable Lasers,xe2x80x9d by Michael G. Littman and Karen Liu, Optics Letters, Vol. 6, No.3, pages 117, 118, March, 1981. A mode-hop-free, Littman cavity laser system with broad-range tuning capabilities is set forth in the publication, xe2x80x9cSynchronous Cavity Mode and Feedback Wavelength Scanning in Dye Laser Oscillators with Gratings,xe2x80x9d by Harold J. Metcalf and Patrick McNicholl, Applied Optics, Vol. 24, No. 17, pages 2757-2761, Sep. 1, 1985. The publication xe2x80x9cScanning Geometry for Broadly Tunable Single-Mode Pulsed Dye Lasers,xe2x80x9d by Guangzhi Z. Zhang and Kohzo Hakuta, Optics Letters, Vol. 17, No. 14, pages 997-999, Jul. 15, 1992, describes a dye laser system capable of continuously tuning a single-longitudinal-mode laser beam over a range of more than 190 cmxe2x88x921 by employing a predefined rotation pivot for the tuning reflector and grating.
Various configurations of grating-tuned, Littman-type, external laser cavity systems capable of providing continuous, broadband, mode-hop-free laser beams have been disclosed in U.S. Pat. No. 5,319,668 to Luecke, U.S. Pat. No. 5,867,512 to Sacher, U.S. Pat. No. 5,771,252 to Lang, U.S. Pat. No. 5,802,085 to Lefevre, et al and the publication xe2x80x9cContinuously Tunable Diode Lasers,xe2x80x9d by Timothy Day, Frank Luecke, and Michel Brownell, Lasers and Optronics, No. 6, June, 1993, pp. 15-17. According to these publications, accurate positioning of the pivot is paramount to obtain continuous, broadband tuning capability over the entire emission bandwidth of the gain medium.
Lowering the lasing threshold for grating-tuned external cavity lasers increases the laser power output in the presence of optical power loss occurring inside the laser cavity due to grating diffraction. A method for reducing power loss was described in the publication, xe2x80x9cLasing Threshold Reduction for Grating-Tuned Laser Cavities,xe2x80x9d by Guangzhi Z. Zhang and Dennis Tokaryk, Applied Optics, vol. 36, No. 24, pages 5855-5858, Aug. 20, 1997. This publication introduced a laser system that utilized potentially wasted optical power in an effective feedback configuration.
Mode-hop-free, broadband tunable lasers have been extensively used in a wide range of applications, including laser spectroscopy, optical metrology, in-situ process monitoring and test and measurement of optical passive components in Dense Wavelength Division Multiplexing, Wavelength Division Multiplexing and optical fiber systems.
The output of grating-tuned, external cavity laser systems in the prior art generally consists of two spectral components: (1) a laser beam; and (2) background light radiation comprising Source Spontaneous Emission (xe2x80x9cSSExe2x80x9d) and Amplified Spontaneous Emission (xe2x80x9cASExe2x80x9d) light radiation. The laser beam is the desired output component and consists of substantially coherent, narrow-linewidth laser light. The SSE and ASE radiation, however, constitutes an undesirable incoherent noise background which is emitted directly by the gain element.
The laser beam component of the laser output couples with the SSE and ASE background radiation component in space and time. Although the SSE and ASE background radiation is usually weak in power as compared to the laser output, it has a significant effect in many sensitive applications including test and evaluation of optical passive components and fibers and Dense Wavelength Division Multiplexing, Wavelength Division Multiplexing and optical fiber data-transmission systems. Consequently, there is a need to filter out SSE and ASE background radiation from the output of grating-tuned, external cavity laser systems to obtain a coherent, narrow-linewidth, noise-free output laser beam.
A few types of grating-tuned external cavity laser systems that could suppress SSE and ASE background noise have been described in the publications, xe2x80x9cUsing Diode Lasers for Atomic Physicsxe2x80x9d, by Carl E. Wieman and Leo Hollberg, Review of Scientific Instruments, vol. 62, pages 1-19, January, 1991 and xe2x80x9cImpact of Source Spontaneous Emission (SSE) on the Measurement of DWDM Componentsxe2x80x9d, by Edgar Leckel et al. These systems insert a beam coupler, usually consisting of an optical flat, into the grating-tuned external feedback cavity, along the laser beam path, between coupler partially reflects the laser beam out of the cavity.
FIG. 2 shows a schematic representation of a tunable laser source constructed by Hewlett-Packard Co. based on the concept described in the above-cited publications. Laser system 200 consists of diffraction grating 210, waveguiding device 232, laser diode 250, tuning reflector 260, beam splitter 292, reflection mirror 294 and optical lens 296.
Laser diode 250 is disposed to generate a laser beam which is incident at a grazing angle upon diffraction grating 210. Beam splitter 292 is located along an optical path between laser diode 250 and diffraction grating 210 such that it intercepts a feedback light radiation component diffracted by diffraction grating 210. Reflection mirror 294 is disposed to intercept a light radiation component diverted by beam splitter 292. Optical lens 296 is disposed along an optical path between reflection mirror 294 and waveguiding device 232.
In operation, laser diode 250 generates a laser beam which is incident on diffraction grating 210 at a grazing angle. Part of this beam is reflected by diffraction grating 210 to provide a conventional laser output (not shown in FIG. 2). Diffraction grating 210 also diffracts a feedback light radiation component, which propagates back into laser diode 250 from the retroreflection of tuning reflector 260. Beam splitter 292 intercepts and partially reflects the feedback light radiation component, thereby giving rise to a diverted light radiation component. The diverted light radiation component consists of a laser beam, an angularly-separated SSE light component and an angularly-separated ASE light component. The diverted light radiation component reflects off reflection mirror 294 and is incident on optical lens 296. Optical lens 296 refracts the incident diverted light radiation while maintaining the angular separation between its three constituent components. Upon refraction by optical lens 296, the laser beam component of the diverted light radiation is coupled into waveguiding device 232 while the angularly-separated SSE and ASE components are filtered out, thereby giving rise to a low-noise laser beam (not shown in FIG. 2).
The laser system described above and shown in the embodiment of FIG. 2 has a number of disadvantages. A disadvantages of the laser system of FIG. 2 is that both the conventional output laser beam and the low-noise laser beam coupled into waveguiding device 232 have reduced optical power due to optical power losses and additional optical dispersion which occur in the laser cavity due to the introduction of beam splitter 292. A further disadvantage of this laser system is that the introduction of beam splitter 292 in the laser cavity modifies the cavity length, and consequently, component positions have to be carefully adjusted to achieve mode-free tuning for the output laser beams. Another disadvantage of the laser system shown in FIG. 2 is that introduction of beam splitter 292 into the laser cavity increases the lasing theshold of the laser cavity, therefore increasing the instability of the laser operation of laser diode 250.
Considering the limitations associated with grating-tuned, external cavity laser systems in the prior art, including the disadvantages described above, there is a need for a grating-tuned, external cavity laser system which can produce a continuously-tunable laser output with suppressed SSE and ASE background noise over the entire laser tuning range and with automatic wavelength and power tracking capability.
In an aspect, the invention relates to an external cavity diode laser system comprising a dispersion unit; a gain element producing coherent light incident upon the dispersion unit, and the dispersion unit dispersing the incident coherent light into dispersed light, the dispersed light comprising a reflected diffraction beam and at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission; a guiding dispersion unit that guides the dispersed light diffracted upon it from the dispersion unit while maintaining an angular separation between the reflected diffraction beam and at least on of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission; and a physical filtering device that physically filters the reflected diffraction beam from the spatially separated at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission guided to the physical filtering device by the guiding unit to produce a low-noise laser beam.
In another aspect, the invention relates to a laser system comprising an external cavity diode laser that emits dispersed light, and the dispersed light comprising a reflected diffraction beam and at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission; a guiding dispersion unit, positioned along the beam path of the reflected diffraction beam; and a physical filtering device positioned along a beam path of the reflected diffracted beam that physically filters the reflected diffraction beam from the at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission to produce a low-noise laser beam.
In still another aspect, the invention relates to a method comprising providing an external cavity diode laser that emits a reflected diffraction beam and at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission; dispersing the reflected diffraction beam a first time along a propagation direction by disposing a dispersion unit in the optical path of the reflected diffraction optical beam; and physically filtering the reflected diffraction beam from the at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission.
In an aspect, the invention relates to an external cavity diode laser system comprising first dispersive means; means for producing coherent light incident upon the first dispersive means, the first dispersive means dispersing the incident coherent light into dispersed light, the dispersed light comprising a reflected diffraction beam and at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission; and physically-filtering means, disposed along a beam path of the reflected diffraction beam, for physically filtering the reflected diffraction beam from the at least one of angularly-separated source spontaneous emission or angularly-separated amplified spontaneous emission to produce a low-noise laser beam.